Mairplot calculates the foldChange (the Y-axis in the plot) from the meanTreated and meanUntreated samples behind the scenes; the comment is explaining that the fold change is plotted as the y-axis even though it's not being explicitly given to the mairplot function.
A mairplot shows differences between two measurements of gene expression by plotting the mean intensities (log scale, x-axis) vs fold change (log intensity ratio, y-axis). The mean intensities are calculated by multiplying the mean gene expression values of the two samples, and transforming to log10 scale. Fold change is plotted as the log2 ratio between the mean expression levels of each sample. If gene Z is expressed 4 times as much in the untreated group, it will have a Y-value of 2. If gene W is expressed half as much in the second group, it would have a Y-value of -1 This makes over & under-expressed genes have the same linear scale on the Y axis.
Data points with largely positive or negative fold changes indicate large changes in gene expression between the two groups. The default cutoff ratio (orange dashed line) is 2, which in log scale becomes +-1. Data points farther to the right tend to have a better signal-to-noise ratio, because the measurements contained higher levels of gene expression in both samples.
Hope this helps,
-Tim
